The bi-partisan commission established by the US Congress to investigate the attacks of 11 September 2001 is due to issue its final report.

High-profile figures including Donald Rumsfeld have testified

During its 20-month investigation, it has interviewed more than 1,000 witnesses, including President Bush himself, and reviewed more than two million documents.

It has already published preliminary findings and the final report, expected to be released on 22 July, will include recommendations on how the government can improve homeland defence.

Q: What is the commission's mandate?

The National Commission on Terrorist Attacks Upon the US was set up by Congress in late 2002 with a mandate to provide a "full and complete account" of the 11 September 2001 attacks and recommendations on how to prevent such attacks in the future.

The commission has been investigating:

Al-Qaeda, its organisation and planning of the attacks

Broader terror networks including how such groups finance their activities

US border, aviation and transportation security

US domestic law enforcement and intelligence gathering

International intelligence gathering, co-operation and analysis

International diplomacy

International and domestic counter-terrorism strategies

Emergency response in the wake of the attacks.

Q: What findings has the commission published so far?

The commission has presented 17 "staff statements", or preliminary reports, during a series of public hearings - all are available to read on the commission website.

The final report will present recommendations for improving homeland defence

The reports have covered issues such as how the hijackers entered the US, details on the four hijacked flights, counter-terrorism operations before 11 September, the performance of the intelligence community and emergency preparedness.

When the final public hearings began on 16-17 June, the commission published three preliminary reports: An outline of the 9/11 plot, an overview of al-Qaeda's role in the attacks and a critical analysis of the US air defence response.

In the latter report, the commission said US air defences were disastrously unprepared for the terror attacks.

The commission's blunt statement that it has found no credible evidence that Iraq and al-Qaeda co-operated in the attacks has attracted much attention.

Iraq's alleged links with al-Qaeda were part of the justification that the Bush administration gave for launching its invasion to topple Saddam Hussein's regime.

Correspondents say the material is building up into a complex picture of missed opportunities - and some of it does not make pleasant reading for the president.

Q: Who is on the panel?

In an attempt to be bipartisan, the panel includes an equal number of representatives from the two major parties in the US - five Democrats and five Republicans. The commission itself has nearly 80 full-time employees.

Republicans on the commission:

Thomas Kean, commission chair, former governor of New Jersey

Fred Fielding, former counsel to the president for Ronald Reagan and member of Bush-Cheney transition team

Slade Gorton, former US senator, Washington state attorney general

John Lehman, former secretary of the Navy under Ronald Reagan

James Thompson, former governor of Illinois

Democrats on the commission:

Lee Hamilton, commission vice-chair, former US representative

Richard Ben-Veniste, chief of the Watergate Task Force of the Watergate Special Prosecutor's Office

Jamie Gorelick, former deputy attorney general of the United States and general counsel of the Department of Defence

Bob Kerrey, former governor of Nebraska and US senator

Timothy Roemer, former US representative

Q: Who has it heard from?

The commission says it has interviewed more than 1,000 individuals in 10 countries. In its first 10 days of public hearings, the commission received testimony from 110 federal, state, and local officials, and experts from the private sector.

The panel's work was thrust into the spotlight in March, with allegations from former White House security expert Richard Clarke that the Bush administration did a "terrible job" confronting terrorism.

In an April round of public hearings, the commission interviewed key figures from the Clinton and Bush administrations including Mr Bush and his vice-president Dick Cheney, former Secretary of State Madeleine Albright, former Secretary of Defence William Cohen, Director of Central Intelligence Agency - until this month - George Tenet as well as the current Secretary of State Colin Powell and the current Secretary of Defence Donald Rumsfeld.

National Security Adviser Condoleezza Rice initially met the commission in private but declined to testify in public.

Democrats and some family members of 9/11 victims responded angrily, accusing the Bush administration of not co-operating fully with the commission.

The White House said this was a separation of powers issue and followed a tradition that senior White House staff do not testify before congressional panels while they are in office.

However, the administration finally yielded to pressure and allowed the public questioning of Ms Rice by the committee.

Former New York Mayor Rudolph Giuliani testified in May, when the commission looked into police and fire department response to the World Trade Center attacks. The panel also heard from Mr Giuliani's successor, Michael Bloomberg, and US Secretary of Homeland Security Tom Ridge.

The commission also met former US President Bill Clinton and former Vice President Al Gore privately in April.

Q: When will it report?

The commission was originally set to report by 27 May, but that deadline was pushed back by two months at the request of the commission to allow it to finish its work.

Republicans, including Speaker of the House of Representatives Dennis Hastert, fought the delay. The report will be released as the presidential campaigns begin in earnest. Through a spokesman, Mr Hastert said he did not want the report to "become a political football in the middle of the campaign".

The final report to President Bush and Congress is now due to be published on 22 July, to avoid competing with the Democratic National Convention, which begins on 26 July.

Q: Has the commission been able to maintain a bipartisan approach?

The panel is taking pains to appear bipartisan, citing lapses by both the Bush and Clinton administrations in its preliminary findings. But its process and findings will be seen through the filter of election-year politics as President Bush seeks re-election with a campaign that highlights his record as a "war-time president" fighting a war on terror.

No matter how even-handed, the two parties are sure to look for political advantage when the final report is published.